As known widely, in the CDMA radio communication system, it is indispensable to perform the transmission power control (especially in uplinks) in order to reduce interference.
In a conventional CDMA radio communication system, the mobile station determines whether to request the base station to increase or decrease the transmission power on the basis of receiving quality of signals from the base station, and then transmits a transmission power control signal representing an increase request or a decrease request to the base station.
An example of such processing is shown in FIG. 1. Each mobile station determines whether the receiving quality is good or not by comparing it with a predetermined threshold, and then transmits the transmission power control signal representing the decrease request when the receiving quality exceeds the threshold, while it transmits one that represents the increase request when the receiving quality is lower.
The base station, which receives such transmission power control signal, controls its transmission power used for the sending mobile station of that transmission power control signal in accordance with the increase or decrease request that is represented in the received signal.
In the conventional CDMA radio communication system, a spreading code is assigned separately for each mobile station. For example, in the example shown in FIG. 1, signals transmitted from Mobile Station #1 are spread by Code #1, while ones from Mobile Station #2 are spread by Code #2.
In such case where a separate spreading code is assigned for each mobile station, orthogonality among the spreading codes becomes weak and the interference increases in the uplink when the number of the mobile stations increases and hence the number of the spreading codes required increases.